Steam power stations or steam power plants are widely known, for example from http://de.wikipedia.org/wiki/Dampfkraftwerk (available on Aug. 11, 2012).
A steam power station is a type of power station for generating electricity from fossil fuels, in which a thermal energy from water vapor is converted into kinetic energy in a steam power plant, i.e. usually a multi-part steam turbine, and is further converted into electrical energy in a generator.
In a steam power station of this type, a fuel, for example coal, is burned in a combustion chamber, as a result of which heat is released.
The heat thereby released is absorbed by a steam generator, i.e. in a power station boiler, consisting of an evaporator (part), referred to only as an evaporator for short, and a superheater (part), referred to only as a superheater for short.
In the evaporator, previously cleaned and processed (feed) water which is fed in there is converted into water vapor/high-pressure steam.
Through further heating of the water vapor/high-pressure steam in the superheater, the steam is brought to the temperature necessary for the “consumer”, wherein the temperature and specific volume of the steam increase. The steam is superheated by guiding the steam in a plurality of stages through heated tube bundles, referred to as the superheater stages.
The high-pressure steam generated in this way further enters the steam power plant or the—mainly multi-part—steam turbine and there it carries out mechanical work while expanding and cooling.
The efficiency of a steam power station or steam power plant increases with the temperature of the steam generated in the power station boiler or in the steam generator of the steam power station.
However, permissible maximum temperature limits of a boiler tube material supplied with the steam in the boiler and the turbine which is intended to be supplied with the steam must not be exceeded.
However, the more precisely the steam temperature can be held at a desired value, the closer the desired value can be to the permissible steam temperature limit, corresponding to the permissible material-related temperature limit, i.e. a correspondingly higher efficiency can be achieved in the operation of the steam power plant.
The steam temperature is controlled, inter alia, by injecting water into the steam line upstream of the steam generator or upstream of the evaporator and the superheater stages via corresponding injection valves of a spray-type desuperheater.
It is also known for the superheater to have a very inert behavior with its large iron masses. An adjustment of the injection valve—and therefore the injected water quantity—has an effect only after several minutes on the steam temperature that is to be controlled.
The time delay in the modification of the steam temperature is not constant, but depends on the current steam mass flow rate.
In addition, the steam temperature to be controlled is strongly influenced by numerous disturbances, such as e.g. load changes, soot blowing in the boiler, change of fuel, etc.
A precise temperature control of the steam is difficult to achieve for these reasons.
To solve this problem, i.e. for a precise and reliable control of the steam temperature, a so-called cascade control for the steam temperature is known.
With this cascade control, two interleaved PI control circuits are set up. An outer, slow PI controller controls the steam temperature at the superheater outlet and specifies a desired value for the steam temperature at the superheater inlet (manipulated variable of the outer, slower control circuit), i.e. following the injection.
With this desired value for the steam temperature at the superheater inlet, steam temperature is controlled at the superheater inlet by an inner, fast PI controller (inner, faster control circuit) which adjusts the injection valve (manipulated variable of the inner, fast control circuit).
Disturbances of the steam temperature at the inlet of the injection can be quickly compensated with this cascade control. The disadvantage of the cascade control is that disturbances which affect the superheater itself can be compensated in the outer, slow circuit only, i.e. with low control quality.
A two-circuit control, which is constructed with a structure identical to that of the cascade control with an outer and inner control circuit, provides a further solution to the problem of a precise and reliable steam temperature control.
However, in comparison with the cascade control having the outer, slower and the inner, faster control circuit, the outer control circuit in the two-circuit control is replaced by a computing circuit.
The desired value for the temperature at the super heater inlet is then calculated by means of the computing circuit in each case on the basis of a superheater model and water/steam table relations so that the required temperature is set at the superheater outlet.
The computing circuit can additionally be provided with differentiating elements which allow an early response to disturbances affecting the superheater.
The disadvantage of the two-circuit control is that a very large amount of time is required for an identification of parameters for the superheater model during a commissioning of the steam power plant.
In EP 2 244 011 A1, a state control is proposed for the steam temperature control problem in the outer control circuit of the cascade or two-circuit control.
In this state control, the temperature of the steam is controlled at the outlet of the superheater with feedback of a plurality of partially non-measurable (medium) states of the steam in the superheater in order to determine a controller setting signal (desired value for the superheater inlet temperature).
However, since this plurality of steam states in the superheater which are used in an algorithm of the state control are not measurable, an observer circuit is required with which the required states are estimated.
The advantage of this state control is that it enables a very fast and accurate response to disturbances affecting the superheater.
However, such an algorithm of the state control responds highly sensitively to changes in a dynamic behavior of a control path in the state control. Although very good control results are achieved e.g. in a load point of the steam power plant, only an insufficient control behavior is achieved under changed operating conditions of the steam power plant.
To solve this problem, EP 2 244 011 A1 then further provides a Linear Quadratic Regulator (LQR) in the state control. This, i.e. the LQR, is a state regulator whose parameters are determined in such a way that a quality criterion is optimized for the control quality.
The quality criterion for the linear quadratic regulation also takes account of the relationship of the parameters, the manipulated variable u and the controlled variable y, wherein the priorities are determined by the Qy and R matrix. The quality value J is determined according to:J(x0,u(t))−∫0∞(y′(t)Qyy(t)+u′(t)Ru(t)dt. 
The static optimization problem for this, which is solved by the linear quadratic regulation, reads (with K as the regulator matrix and x0 as the initial state):
            min              u        ⁢                  {          t          }                      ⁢                  ⁢          J      ⁡              (                              x            0                    ,                      u            ⁡                          (              t              )                                      )              =                    min                              u            ⁢                          {              t              }                                =                                    -              Kx                        ⁢                          {              t              }                                          ⁢                          ⁢              J        ⁡                  (                                    x              0                        ,                          u              ⁡                              (                t                )                                              )                      =                  min        K            ⁢                          ⁢                        J          ⁡                      (                                          x                0                            ,                              -                                  Kx                  ⁡                                      (                    t                    )                                                                        )                          .            
In EP 2 244 011 A1, a Kalman filter, which is similarly designed according to the LQR principle, is used as an observer. The interplay of the LQR with the Kalman filter is referred to as the LQG (Linear Quadratic Gaussian) algorithm.
However, the LQG method used according to EP 2 244 011 A1 relates to linear regulation problems, whereas the injection mass flow, as the final manipulated variable of the inner control circuit, acts in a non-linear manner on the temperature controlled variable.
Through a consistent conversion—furthermore also provided according to EP 2 244 011 A1—of all temperature measurement values and desired temperature values into enthalpies, a linearization of the regulation problem is achieved, since a linear relation exists between the injection mass flow and the steam enthalpy. The conversion of temperature into enthalpy is effected here by means of corresponding water/steam table relations using a measured steam pressure.
Through this linearization in EP 2 244 011 A1, a very robust control behavior is achieved, i.e. the control quality no longer depends on the current operating point of the steam power plant.
The calculation of a feedback matrix in the state regulator (regulator matrix) and also the corresponding feedback matrix in the observer (observer matrix), correspondingly constructed according to the LQR principle of the state regulator, by which the regulator is finally represented, is carried out continuously online in EP 2 244 011 A1, in each case using current measurement values.
The regulator in EP 2 244 011 A1 thus adapts continuously to the actual operating conditions of the steam power plant. For example, a load-dependent change in the dynamic superheater behavior is thereby automatically taken into account.
An increase in the robustness of the control algorithm is thus achieved in EP 2 244 011 A1 through this online calculation of the feedback matrix.
Disturbances directly affecting the superheater are expressed in that a temperature rise, i.e. a relation of the enthalpies between the superheater outlet and inlet, changes.
EP 2 244 011 A1 therefore provides here that not only the states or the temperatures along the superheater are estimated, but additionally the disturbance or a disturbance parameter is defined as a further state and is estimated using the observer.
A very fast, accurate but simultaneously robust response to corresponding disturbances is thus possible.
Due to the fact that this control algorithm according to EP 2 244 011 A1 is very robust due to the described measures (linearization, online calculation, disturbance parameter estimation), only very few parameters need to be set during the commissioning of a steam power plant. The commissioning time and cost are therefore substantially reduced.
However, the state control constructed in this way with LQG, i.e. with a state regulator and observer according to the LQR principle, according to EP 2 244 011 A1, also has various disadvantages.
The online calculation of the regulator and observer matrix is associated with a very substantial computing time and storage space requirement. It can therefore no longer run simultaneously with other automation functions on a standard automation processor.
It is thus necessary to provide additional automation processors which are, however, very expensive, or to use one or more separate PC modules, which are coupled into a control technology system of the steam power station.
This applies particularly in view of the fact that calculations of this type must be carried out for each individual steam temperature control circuit (e.g. around 20 circuits in a large coal-fired power plant).
The use of LQG control, as proposed according to EP 2 244 011 A1, is therefore associated with an additional cost for the hardware and corresponding spare parts procurement.
Although the observation of the heat flow as a disturbance parameter acting on the superheater is advantageous, it cannot overcome the difficulty that the regulator responds to changes in the fuel mass flow only when this control intervention has already taken effect on the steam temperature at the superheater outlet.
In parallel with the LQG regulator, a derivative element must therefore be used which ensures that when the fuel mass flow is adjusted, the injection mass flow is simultaneously adjusted, so that the effect on the steam temperature can be minimized.
A derivative element of this type must be parameterized in plant tests, which is a time-consuming and costly process.